Ever increasing demands are being placed on designers to provide near optimum propulsion systems for advanced commercial and military vehicles of all types. Propulsion systems are needed which provide improved fuel economy, better maneuverability, less operator skill and fatigue, more adaptability to load, faster acceleration, greater safety, increased engine life and improved maintainability.
A vehicle propulsion system consists of a power source, or an engine, a power transfer device or transmission, and controls which couple the engine and the transmission for smooth and efficient vehicle control. If well designed, the transmission can provide the proper match between engine and load in order to maximize fuel economy and overall performance, and to eliminate stalling of the engine under maximum transmission loading conditions.
There is an increasing trend toward the use of hydrostatic and split path hydromechanical transmissions for high performance vehicles. The overall efficiency of a vehicle propulsion system can be greater with a hydrostatic transmission (HST) than with a straight mechanical transmission under certain duty cycles, even though the HST is the lesser efficient transmission of the two. The reason for this apparent paradox lies in the infinitely variable speed capability of the HST and its resulting superior controllability.
The simplest and most common way to control a vehicle propulsion system with an HST is to govern the engine to a nearly constant speed and control the transmission manually. Except for inherent feedback provided by the operator, the transmission control is open loop. This method is adequate if the system's inefficiencies are primarily those of the engine, as is the case with the straight mechanical transmission. However, when the relatively low efficiency HST is used, instead of a mechanical transmission, the overall system efficiency is less with this open loop design.
The HST is especially suitable for closed loop control. With proper closed loop control the engine can be forced to operate at its best efficiency under all throttle and load conditions. This invention is directed to a type of system which involves closed loop control with an HST having an infinitely variable transmission ratio.
In the prior art there have been a number of different types of control systems, some semiautomatic, that is, partially manual and partially automatic, and some automatic, but each of them were dependent upon two or more variables chosen from a wide group of variables. The most common parameters which have been used to control the engine have been the speed and throttle position. These are normally handled by a governor type of mechanism in which the actual throttle position at the engine is a function of a desired throttle position produced by the governor and a manual throttle position produced by the operator.
Because of the basic engine characteristics of the internal combustion engine, the relationships between the various parameters such as speed, vacuum, throttle position, load ratio, etc. taken in pairs is rarely, if ever, a linear relationship. Thus, the control system must provide means for converting one variable to another, even where the characteristic is nonlinear. Therefore, in the choice of variables which are used to control the engine, it is important to choose two variables which are basically important in the control of the engine. Also the two variables should be related to each other in a certain characteristic function which can be duplicated by physical apparatus.